The development of electrostatic latent images with toner particles is well known. Over the years, the level of sophistication achieved in the field of electrostatic latent image development systems has been remarkable. For example, slow and laborious manual systems commercialized in the late 1950s have evolved into elegant high speed development systems creating as many as three copies per second. Consequently, the performance standards for toners during the evolution of electrostatography have become increasingly stringent. In the early manual development systems, toner and carrier particles were merely moved over an imaging surface bearing an electrostatic latent image by hand, tilting a tray containing the imaging surface, toner and carrier particles. However, in recent years, toner particles are automatically recycled many thousands of times over imaging surfaces moving at extremely high velocities. Thus, durable toner materials are required to withstand the physical punishment of vigorous, prolonged and continuous use. Moreover, toner particles deposited in image configuration must now be fused in extremely short periods of time.
Due to the size limitations of electrostatic copying and printing machines, the fusing path must be relatively short. When one attempts to increase the heat energy applied to deposited toner images for fusing purposes within the constraints of a limited fusing path to achieve adequate fixing at higher rates, one approaches the charring or kindling temperature of the substrate bearing the toner image. Attempts to shorten the fusing path by utilizing flash fusing techniques often result in the formation of noxious fumes due to decomposition of components in some toners. Further, the cost and availability of energy to operate an electrostatographic imaging system is of increasing concern to users. In addition, toner materials must possess the proper triboelectric charging properties for electrostatic latent image development and must not agglomerate during storage and transportation. Thus, there is a great need for an improved toner having stable electrical and physical properties which can endure the harsh environment of high speed electrostatographic copiers and printers and which can also be fused at lower temperatures utilizing less energy.
It is well known that electrostatic latent images can be developed with toner compositions which are comprised of a melt blend of toner resin and pigment particles. In such systems, negatively charged toner particles are generally selected for the development of positively charged electrostatic latent images. However, in recent years, the use of positively charged toner compositions containing charge enhancing additives for the purpose of imparting positive charge to toner resin particles has become more popular. These positively charged toner compositions are particularly useful for causing the development of negatively charged electrostatic latent images formed on layered organic photoreceptor imaging members. Examples of positively charged toner compositions useful for causing the development of negatively charged electrostatic latent images are disclosed in U.S. Pat. No. 4,298,672, U.S. Pat. No. 4,338,390 and U.S. Pat. No. 4,469,770.
Certain copolymers of styrene and butadiene have been developed which meet the demanding requirements of positively charged toner compositions. Such styrene-butadiene copolymers can be made by various techniques with emulsion polymerization being the most common. However, there are a number of traditional drawbacks associated with utilizing emulsion polymerization in preparing such toner resins which are utilized in preparing toners designed to build stable charge. For instance, undesirable residual contaminants are typically present in toner resins made by emulsion polymerization. In many cases, these residual contaminants have a very detrimental effect on the performance characteristics of the toner resin.
Rosin acids and fatty acids are commonly utilized as emulsifiers in preparing toner resins by emulsion polymerization. The presence of residual rosin acids and residual fatty acids in toner resins limits their ability to build stable electrical charges. The coagulants utilized in recovering the resin from the aqueous emulsion are also generally present as residual contaminants in such toner resins. The presence of ash from the coagulants also limits the ability of toners made utilizing such resins to build a stable charge. For these reasons, emulsion polymerization has typically been considered to be inferior to solution polymerization and suspension polymerization techniques for synthesizing such toner resins.
U.S. Pat. No. 5,247,034 discloses the utilization of amino acid soaps in the synthesis of toner resins and circumvents some of the problems associated with utilizing rosin acid soaps or fatty acid soaps. By virtue of the fact that such emulsions can be coagulated without the utilization of salts, the resins made by the process disclosed in U.S. Pat. No. 5,247,034 exhibit low levels of residual ash. This is advantageous in that the presence of ash reduces the level of charge which can be realized. As a result of the low level of ash, the toner resin made utilizing the amino acid soap exhibits excellent resistance to moisture sensitivity and adsorption. This feature gives toners made from these resins better electrical charge stability compared to resins made from other soaps since adsorbed moisture is known to neutralize electrical charges. However, toners made with resins synthesized utilizing the technique of U.S. Pat. No. 5,247,034 have low adhesion characteristics to paper.